Auditory function is dependent on the formation of a functional cochlea, which includes the auditory sensory epithelium, referred to as the organ of Corti, and the associated spiral ganglion neurons that provide afferent neuronal innervation to the organ of Corti. The organ of Corti contains at least 6 different types of cells including mechanosensory hair cells and non-sensory supporting cells. Hair cells, supporting cells and spiral ganglion cells are all derived from a limited region of the otocyst, an embryonic structure that develops adjacent to the hindbrain. Other regions of the otocyst normally go on to develop as non-sensory structures within the inner ear. Existing data suggests that individual cells become specified to develop as either neuroblasts that will give rise to the afferent neurons of the cochlea or a population of prosensory cells that will then become subdivided into hair cells and supporting cells. While recent work has begun to identify some of the molecular signaling pathways that regulate these developmental events, our understanding is still fairly limited. During the previous year, different members of the laboratory have examined several different aspects of these developmental processes. First, we demonstrated that three different transcription factors, Neurog1, Neurod1 and Sox2 are both required for development of the spiral ganglion and can induce non-sensory regions of the inner ear to develop as neurons. Previous work from the laboratory had demonstrated that a different transcription factor, Atoh1, could induce similar non-sensory cells to develop as hair cells. Interestingly, the ability of these cells to develop as neurons is lost during the late embryonic period in mice while the ability of these cells to develop as hair cells persists for a considerably longer period of time. In a separate series of experiments the role of a different transcription factor, Pou3F4, in neuronal development was examined. Mutations in Pou3F4 lead to deafness in mice and humans even though this gene is not expressed in either hair cells or ganglion cells. Te determine the basis for the auditory defect, we examined the effect of mutations in Pou3F4 on neuronal innnervation. Results indicate that Pou3F4 is expressed in the mesenchymal cells that surround ganglion cell neurites as they extend towards the hair cells and in Pou3F4 mutants this extension is disrupted. In a separate series of experiments we have examined the developmental potential of cells that normally express the Atoh1 transcription factor. Previous results had suggested that Atoh1 is only expressed in cells that will develop as hair cells. However, using a genetically based lineage tracing method we were able to demonstrate that some cells the initially express Atoh1 will go on to develop as supporting cells. These results suggest that other factors contribute to the determination of which Atoh1-positive cells will go on to develop as hair cells. Previous work from the laboratory demonstrated that Rbpj, a component of the notch pathway, is required for the formation of prosensory cells. One of the results of those experiments indicated that the insulin growth factor (IGF) signaling pathway was a target of Rbpj. In a series of follow up experiments we have demonstrated that perturbation of Igf signaling prevents prosensory cells from expression Atoh1, and therefore, from differentiating as hair cells. In a new series of experiments within the laboratory we have examined role of cochlear outgrowth on patterning of individual hair cells and supporting cells within the organ of Corti. If cochlear outgrowth is inhibited, then cellular patterning is disrupted, suggesting that developing hair cells actively move or rearrange during their formation. To examine this more closely, we have begun generating time-lapse movies of migrating hair cells. Results indicate for the first time active migratory motion of developing hair cells. Finally, several recent studies have identified a new class of genetically-based syndromes, that arise from mutations in genes that are associated with cilia or basal bodies. These syndromes, referred to as ciliopathies, are characterized by a number of different biological defects, including, in some cases, hearing loss. In collaboration with the laboratory of Anand Swaroop at NEI, we have examined the effects of two known ciliopathy genes, Cep290 and Mkks, in ciliary formation in both the eye and ear. Deletion of Mkks leads to an absence of cilia that is surprisingly corrected in the presence of some mutated forms of Cep290. These results have intriguing implications for both basic biological studies of cilia formation as well as for existing patient populations.